In tests we now automatically remove any handlers registered with
`ulif.diceware` loggers.

The path to wordlist files is hardcoded in the generated sphinx
documentation and because it may differ depending on how the package is
installed this creates different problems:

  - the path hardcoded in the online api documentation may differ from
    the one in each installation, leading to incorrect information
    provided to the user of the api.

  - packaging diceware documentation to specific operating systems may
    generate unreproducible packages.

This commit replaces the variable WORDLISTS_DIR by a function called
get_wordlists_dir() that delays the construction of the path instead of
doing it during module loading.

We want that kind of fake input for other tests as well and will
turn it into a pytest fixture.

With the new config module in place we must make sure, that any user
maintained .diceware.ini files do not interfere with tests.

The fixture sets $HOME for all tests automatically.

Use conftest module for fixtures we need in several test modules.